Which row correctly shows forces that are acting at P and Q?

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26) The diagram shows a rectangular gate with a diagonal cross bar. Without the diagonal bar, a rectangular gate would tend to deform into a parallelogram due to its own weight or external forces, causing it to droop or sag. The diagonal cross bar forms two triangles, which are rigid shapes, thereby preventing this deformation.

Analyze the function of a diagonal cross bar in a gate.
A rectangular frame is not rigid and can easily change shape into a parallelogram. Adding a diagonal bar divides the rectangle into two triangles, which are rigid structures. This rigidity prevents the gate from deforming.
Evaluate the options.
- A: buckling refers to a sudden structural failure under compression, not the primary deformation prevented here.
- B: dismantling means taking apart, which is unrelated.
- C: sagging refers to the downward drooping or deformation of a structure under its own weight or load. This is precisely what the diagonal bar prevents.
- D: shearing is a type of stress, but the overall effect prevented is the change in shape leading to sagging.
The correct option is C.

27) The diagram shows a loaded cantilever, which is a beam fixed at one end and loaded at the other. When a cantilever bends downwards due to a load, the material on the upper side of the beam is stretched, while the material on the lower side is compressed.
Identify the forces acting on the upper surface (P).
As the cantilever bends downwards, the upper surface (P) is elongated or stretched. This stretching force is known as tension.
Identify the forces acting on the lower surface (Q).
As the cantilever bends downwards, the lower surface (Q) is shortened or squeezed together. This squeezing force is known as compression.
Match the identified forces with the given options.
- A: P: compression, Q: tension - Incorrect.
- B: P: shear, Q: compression - Incorrect.
- C: P: tension, Q: compression - Correct.
- D: P: tension, Q: shear - Incorrect.
The correct option is C.

The final answers are $\boxed{\text{26: C, 27: C}}$ .

Which row correctly shows forces that are acting at P and Q?

Answer

Step-by-step solution

Analyze the function of a diagonal cross bar in a gate.

Evaluate the options.

Identify the forces acting on the upper surface (P).

Identify the forces acting on the lower surface (Q).

Match the identified forces with the given options.

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